Hydraulic tappet

Abstract: A hydraulic tappet 1 includes a cam sliding portion 2, a skirt portion 3 and a spring holding portion 4. At least the cam sliding portion 2 is made of ceramic and oil is held in the spring holding portion 4. There is thus provided a hydraulic tappet capable of efficiently using motive power from a crank shaft, reducing transmission loss and improving the efficiency of the engine.

Description: The present invention relates to a tappet for a valve mechanism for an internal combustion engine, for example, an engine for an automobile, and more particularly to a hydraulic tappet capable of improving the efficiency of the engine for an automobile. An internal combustion engine, such as an engine for an automobile, has a tappet or a rocker arm disposed at an end of a valve shaft to transmit the kinetic force of a cam to a valve mechanism. Thermal expansion or thermal shrinkage of a cylinder head or a valve occurring at the time of the operation and incomplete seating of the valve occurring due to wear of the valve or a valve seat are prevented and the kinetic characteristics of the valve system are improved by disposing a hydraulic or mechanical adjustment mechanism at the location at which the tappet or the rocker arm is in contact with the end of the valve shaft. Fig. 3 illustrates an example of a conventional tappet 10 of a hydraulic adjustment type. If a cylinder head (omitted from the illustration) or a valve 18 undergoes thermal expansion or thermal shrinkage, the hydraulic pressure in an oil reservoir 16 disposed in the metal tappet 10 is used to adjust the position of the spring portion of the tappet for pressing the valve so that the transmission characteristics from a cam 17 are compensated. As shown in Fig. 3, an upwardly projecting annular flange 20 of the base wall of the tappet receives a cup-shaped member 21 which is fixed on the end of the stem or push-rod of the valve 18. Within the cup-shaped member 21 there is a sliding piston member 22 upwardly biased by a spring 23. When the cam sliding surface 12 is pushed down by the cam, the hydraulic pressure in the reservoir 16 is increased, and the valve stem is pushed down via the cup-shaped member 21 to open the valve. The oil in the reservoir 16 is not change during the operation of the tappet. However, a conventional tappet of the foregoing hydraulic adjustment type involves generation of excessively large frictional force when the cam 17 slides on the cam sliding surface 12 of the metal tappet 10 as shown in Fig. 3. As a result, there arises a problem in that the crankshaft must bear an excessively heavy load, and therefore the efficiency of the engine deteriorates. Moreover, the employed mechanism, in which oil is enclosed in the tappet, means that the oil reservoir occupies a large part of the inside of the tappet, and the weight of the tappet is increased excessively, so that the transmission loss occurring due to the vertical motion of the valve 18 is large. While recently study has been made to make the valve itself of ceramic material, in order to reduce its weight thereby to make smooth the movement of the valve so as to improve the efficiency of the engine, use of the conventional metal tappet encounters a problem that the weight of the valve mechanism cannot satisfactorily be reduced, and accordingly the full desired effect of using the ceramic valve cannot be obtained. EP-A-225 096 describes a tappet of the general type shown in Fig. 3. It is suggested that ceramic materials or cermets may be employed for the tappet. The present invention is directed to overcome the foregoing problems experienced with the conventional technology, and therefore an object of the same is to provide a hydraulic tappet capable of efficiently using the force transmitted from the crank shaft, therefore reducing the transmission loss and improving the efficiency of the engine. The present invention provides a hydraulic tappet for a valve mechanism of an internal combustion engine, as set forth in claim 1. In the present invention the cam sliding portion, the skirt portion and the spring holding portion of the hydraulic tappet are made of a ceramic material. Therefore, the total weight of the tappet can be reduced, and the ratio of the power loss occurring during transmission from the crank shaft can be lowered so that the transmission characteristics can be improved. In particular, the arrangement that the cam sliding portion is made of a ceramic material exhibiting superior sliding characteristics to those of metal hitherto used enables the frictional force between a cam and the cam sliding portion, which is generated when the cam is rotated, to be reduced. Furthermore, the oil supply port is formed in the side wall of the tappet to supply oil from outside and oil is reserved in the spring holding portion. In this way the proportion of the oil reservoir occupying the tappet can significantly be lowered, compared with the structure of Fig. 3. Therefore, the total weight of the tappet can be reduced to about two-third to half of that of the conventional tappet, and accordingly the transmission characteristics from the crank shaft can be improved so that the efficiency of the engine can be improved. The supply and refreshment of oil in the spring-holding portion can take place during operation, from the oil around the tappet in the engine. This changing of the oil increases the life of the tappet compared with the tappet of Fig. 3. The present invention will now be described through embodiments with reference to the drawings. Fig. 1 is a cross-sectional view which illustrates an embodiment of a hydraulic tappet according to the present invention. Fig. 2 is a cross-sectional view which illustrates another embodiment of a hydraulic tappet according to the present invention. Fig. 3 is a cross-sectional view which illustrates a conventional hydraulic adjustment type tappet. First Embodiment Fig. 1 is a cross-sectional view which illustrates an example of a hydraulic tappet according to the present invention. The hydraulic tappet 1 has a cam sliding portion 2, a skirt portion 3 and a spring holding portion 4, all of which are made of ceramic, resulting in weight reduction compared with a structure in which they are made of metal. Since ceramic can exhibit excellent sliding characteristics, the frictional force can significantly be reduced between the cam sliding portion 2 and a cam (omitted from Fig. 1). As a result, the efficiency of the engine can be improved. Although the type of the ceramic material to be employed is not limited particularly, it is exemplified by Si 3 N 4 and sialon. In particular, Si 3 N 4 exhibits excellent friction characteristics with chilled cast iron, which is the material of the cam, and is preferably employed. Further, the structure is formed in such a way that an oil supply port 5 is formed as a conduit between the skirt portion 3 and the spring holding portion 4 to be supplied freely with oil from outside as the tappet 1 is operated and that oil is held in only the spring holding portion 4. The spring holding portion 4 which is an annular skirt projecting downwardly from the top wall of the tappet acts to guide and hold the parts 21,22,23 (shown in Fig. 3 and omitted from each of Figs. 1 and 2) and serves as an oil reservoir. The tappet therefore has the same hydraulic adjustment function as the tappet of Fig. 3. Pressure is maintained as required in the oil reservoir due to the oil pressure outside the narrow bore of the port 5. Therefore, as contrasted with the conventional hydraulic tappet arranged in which the oil is held in the tappet by forming an oil reservoir to surround the spring holding portion (see Fig. 3), this embodiment has the arrangement that only the spring holding portion 4 serves as the oil reservoir and oil required is supplied from outside, so that the oil is changed during operation. As a result, the size of the oil reservoir can be reduced, and therefore the total weight of the tappet can be reduced in addition to the weight reduction realized by the ceramic material. The oil change increases the life of the tappet. Therefore, the efficiency of the engine can further be improved. Since the tappet 1 is made of ceramic, excellent heat resistance and corrosion resistance can be exhibited as compared with the conventional hydraulic tappet. An integrated-structure ceramic tappet can easily be manufactured by an injection molding method or a slip casting method or the like. Further, it can be shaped into a nearly final shape which requires substantially no time in the machining after forming. In the other Figure and the description below, substantially the same elements as the foregoing elements are given the same reference numerals as in Fig. 1 and description of them and their functions will be omitted. Second Embodiment Fig. 2 illustrates another example of the hydraulic tappet according to the present invention. In this embodiment, the oil supply port 5 is formed of a metal pipe. Although a ceramic usually requires a great many processes to be manufactured and machined in detail, for example, forming an aperture, the arrangements that the oil supply port 5 is formed of the metal pipe facilitates machining. Therefore, the cost can be reduced. The metal pipe can easily be connected to the body of the ceramic tappet by press-fitting or brazing. Further, the weight can be reduced in this case as compared with the conventional arrangement that the tappet is fully made of metal material. Fig. 2 shows that there is a slot 2a in the top wall and side wall of the tappet; however the top of the spring holding portion 4 is closed. Although the present invention has been described through the embodiments, the present invention is not limited to the foregoing embodiments. For example, the present invention can be applied to a variety of internal combustion engines as well as to the engine for an automobile. By combining it with a ceramic cam and/or a valve, the efficiency of the engine can further be improved. As described above, according to the present invention, an oil supply port is used and a ceramic material is used to form the tappet so that there can be provided a hydraulic tappet capable of efficiently using the motive power from the crank shaft, reducing the transmission loss and improving the efficiency of the engine.

Claims: A hydraulic tappet for a valve mechanism of an internal combustion engine, said hydraulic tappet comprising component elements in the form of a cam sliding portion (2), a skirt portion (3) and a spring holding portion (4) projecting downwardly below the cam sliding portion (2), wherein all three of said component elements are formed as a unitary ceramic body, and said spring holding portion (4) has an oil-holding space, characterized by an oil supply port (5) that extends from said oil-holding space of the spring holding portion (4) to said skirt portion (3) to provide a conduit for oil to pass between said oil-holding space and the exterior of the tappet. A hydraulic tappet according to claim 1, wherein said oil supply port (5) is a metal pipe. A hydraulic tappet according to claim 2, wherein said metal pipe is disposed in a slot (2a) formed in the tappet, the slot (2a) opening to the cam sliding portion (2) and skirt portion (3) of the tappet.

Inventors: KAWASAKI KEIJI, MIWA SHINICHI, KAWASAKI, KEIJI, MIWA, SHINICHI, KAWASAKI, KEIJI GURORIASU CHIRYU 3, 704
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1994
Main classification: F01L1
IPC codes: F01L1, F16H53